Hitherto, several attempts have been made to achieve a method for developing an electrostatic latent image by turning liquid ink into spray ink utilizing ultrasonic wave or static electricity and selectively applying it onto the electrostatic latent image with electrostatic power.
For example, in the art described in Japanese Patent Publication (examined) No. 52-7936 and Japanese Patent Publication (examined) No. 55-20230, a concave vibrator excites an ultrasonic wave in developing solution, and the developing solution is jetted from an opening portion of a container for storing the developing solution. Particularly, in the art described in Japanese Patent Publication No. 55-20230, the excitation by an ultrasonic wave is supplemental and an electric field from the electrostatic latent image causes a mist to generate from the developing solution. In the art described in Japanese Patent Publication (unexamined) No. 4-337772, a surface acoustic wave generated in a piezoelectric member forming a comb electrode jets a developing agent existing in the edge of the piezoelectric member.
Further, in the art described in Japanese Patent Publication (unexamined) No. 5-333703, standing-wave vibration of a transverse wave is excited in a member connected with a piezoelectric vibrator, thereby turning an ink supplied thereon into a mist, and the mist is charged by a grid electrode. Further, in the art described in Japanese Patent Publication (unexamined) No. 9-319229, a charged ink mist is sent to a developing position by a fan and is applied onto the electrostatic latent image.
However, in the Japanese Patent Publication (examined) No. 52-7936, the ultrasonic wave has a frequency of 20 to 30 kHz, and diameter or size of ink particles is large and their momentum is large. Therefore, developing solution is applied onto portions where any latent image does not exist (causing thereby so-called "scumming" and "fog"). As any particular jetting spout is not provided therein, it is impossible to efficiently decrease the particle size (particle diameter) by increasing the frequency of the ultrasonic wave. Furthermore, in the art described in Japanese Patent Publication No. 55-20230, it is impossible to increase generation amount of mist attracted with the power of the electric field of the electrostatic latent image, and it is difficult to attain a record of high speed and high density.
In the art described in Japanese Patent Publication (unexamined) No. 4-337772, manufacturing a mechanism for generating a surface acoustic wave requires a high processing accuracy, and the apparatus is not provided with any jetting spout. It is therefore suspected that the particle size of the developing solution varies widely.
In the art described in Japanese Patent Publication (unexamined) No. 5-333703, the apparatus becomes complicated and large in order to circulate ink with a pump and install the grid electrode. Further, since the apparatus uses a standing wave of a transverse wave, when particle size is decreased by shortening the wavelength, it is difficult to uniformly decrease them, eventually resulting in occurrence of scumming and. Moreover, depressed portions on the grid electrode are stained with ink, and periodical cleaning is necessary.
Particularly in Japanese Patent Publication (unexamined) No. 9-319229, the apparatus becomes complicated and large in order to convey ink mist with a fan and install a dielectric charged electrode. Moreover, it takes a lot of time from the generation of the ink mist until it is ready for developing because the ink mist is conveyed with the fan. It is very difficult to control density of the mist on the basis of the printing density and control start/stop for jetting the mist.